The news sounds great: More COVID-19 vaccine doses have been administered than the number of US Covid-19 cases reported during the entire pandemic.
More than 26.3 million cases have been reported over the past year, according to Johns Hopkins University.
But that doesn't mean the pandemic is going away. Here's why:
-- The number of coronavirus infections may be 4 times higher than the number of cases actually reported, researchers say.
"Even after adjusting for underreporting, a substantial gap remains between the estimated proportion of the population infected and the proportion infected required to reach herd immunity," the researchers wrote.
-- New, highly contagious variants are spreading across the US, threatening to reinfect people who've already had the coronavirus.
-- Only about 1.8% of the US population has received both doses of a COVID-19 vaccine. And it'll be at least several months until most Americans can get vaccinated.
'This doesn't appear to be great news for vaccine efficacy'
While viruses mutate all the time, a few variants, in particular, are worrying scientists:
All three of those variants have been found in the US, and the B.1.1.7 strain has been detected in at least 32 states.
Scientists have now discovered a mutation in at least 11 samples of the B.1.1.7 strain that might escape antibody protection, according to a report Monday by Public Health England.
The mutation, called E484K, was already part of the genetic signature of variants linked to South Africa and Brazil.
A new lab study found that antibodies from vaccinated people were less effective at neutralizing a synthetic virus resembling samples of B.1.1.7 that had developed an E484K mutation
"This doesn't appear to be great news for vaccine efficacy," said Joseph Fauver, an associate research scientist in epidemiology at the Yale School of Public Health.
Experts say this news could mean the B.1.1.7 strain, already known to be more transmissible, might be somewhat resistant to the protection given by vaccines, or more likely to cause reinfection among people who were previously infected.
Previous studies suggest that E484K may be the key reason why certain vaccines appear less effective in South Africa. Lab research has also shown that antibodies appear less able to bind and neutralize spike proteins arising from the mutation.
Recovered Covid-19 patients could still get reinfected
"We need to get as many people vaccinated as quickly as we possibly can," said Fauci, director of the National Institute of Allergy and Infectious Diseases.
"Even though there is a diminished protection against the variants, there's enough protection to prevent you from getting serious disease, including hospitalization and deaths."
He said health experts in South Africa have noticed such a high rate of reinfection that previous infection did not appear to protect people.
Fast, widespread vaccinations could help prevent variants from becoming more dominant.
"Viruses cannot mutate if they don't replicate," Fauci said.
"And if you stop their replication by vaccinating widely and not giving the virus an open playing field to continue to respond to the pressures that you put on it, you will not get mutations."
Pfizer to deliver 200 million vaccine doses 2 months earlier than planned
The maker of one of two vaccines currently administered in the US confirmed Tuesday that it expects to deliver 200 million doses to the US by the end of May.
Pfizer was originally scheduled to deliver the 200 million doses by July 31. But CEO Albert Bourla said last week he expects the company's production to be ahead of schedule by two months.
"In the US, we had promised to provide 100 million doses by the end of the first quarter and we will be able to provide 120 right now," Bourla said last week.
"The same is with second quarter. We were planning to provide them all the way to 200 million doses by the end of the second quarter, actually beginning of the third. Right now, we will be able to provide the 200 million doses two months earlier."
The Biden administration has announced it will be purchasing an additional 100 million doses from the company.
Pfizer said it had supplied 20 million doses to the US as of Sunday.
Both Pfizer's vaccine and the vaccine made by Moderna require two doses, spaced 21 days and 28 days apart.
People previously infected might only need 1 shot
Those who already had coronavirus and hope to avoid reinfection might only need one dose of a vaccine, according to a study posted Monday.
People who were previously infected tended to have antibody levels that were at or above those of people who had gotten both doses but never been infected, wrote the authors of the study, which has not been peer-reviewed.
The authors said "changing the policy to give these individuals only one dose of vaccine would not negatively impact on their antibody titers, spare them from unnecessary pain and free up many urgently needed vaccine doses."
A titer is a measurement of the amount or concentration of the antibodies found in a person's blood, according to the US National Library of Medicine.
"Ongoing follow-up studies will show whether these early differences in immune responses are maintained over time," the authors wrote
The CDC says people should be vaccinated even if they had COVID-19 since it's not yet clear how long antibody protection lasts.
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